PSPP is a free SPSS "clone." (See http://freestatisticalsoftware.com/pspp-overview/ for more details). I'm thinking of using it in a business Research Methods class rather than Excel or SPSS. I'd like to hear about others' experiences with PSPP.
The last I checked, pspp had only a limited number of types of analyses, so you'll want to check the documentation to see if it is sufficient for your needs... I tried using it a couple of years ago, and the program kept crashing, so I didn't get too far.
more or less the software is ok for basic and simple analysis. I have used it to teach undergraduate students and it was somewhat useful as it was free and the students do not need to buy it.